The present invention, realized at the Office de la Recherche Scientifique et Technique Outre Mer 24, rue Bayard 75008-PARIS, relates to a trawl-net apparatus with controlled opening and closing, able to be used particularly for selectively gathering pelagic micro-necton, as well as for gathering planktonic fauna.
The term micro-necton, in oceanology, encompasses active organisms (these are mostly crustaceans, cephalopoda, fish) whose size is between 1 and 10 cm and which can swim at peak speeds of about 2 to 5 knots. The gathering is called discrete, because the trawl-net must only be fishing at the desired time or in the desired place, and not operating to gather micro-necton inopportunely, in particular while the trawl-net is going down and/or going up; it is necessary then for this trawl-net to have controlled opening and closing features.
The control of this opening or closing offers in itself no difficulty; there exists on the market devices responding to the pressure, for example by a piston shearing a gauged stud, and clock movements for control depending on the time.
On the other hand, there have existed for a long time trawl-nets for gathering (non discretely) micro-necton; by way of example, such a trawl-net may have at its mouth an opening of the order of 9 m.sub.2, with, in front of the mouth, a dihedral diving panel having corresponding dimensions; it is then very difficult to open and then close this mouth while the trawl-net is trawled at depth. Certain trawl-nets of same size but of different design have indeed a mouth with controlled opening and closing, but such trawl-nets are difficult to use and slow, which makes them less efficient for fishing and only selects the smallest size organisms of micro-necton.
In the present state of the known technique, it has been proposed to divide the mouth of the trawl-net by a middle dividing wall made from net, which is applied first of all at the bottom, then at the top of the net; but the reverse movement is not provided for and cannot be so because of the hydrodynamic thrust on the dividing wall; thus, the trawl-net is indeed closed while it is going down, but it remains open when coming up, contaminating the deep harvest.
Another proposal is to open and close the collector, which has a small cross-section; but this collector produces immediately upstream, that is to say at the bottom of the trawl-net, a local increase of the hydrodynamic pressure, which deforms this bottom into a collecting pocket on the walls of which the organisms captured by the mouth flatten themselves instead of sliding into the collector; the selection obtained is thus illusory.